The package managers you cite are really just a choice of front-ends to an underlying packaging system. For example, yum is a front-end to RPM, and others exist (smart, zypper, apt4rpm, and so on). Similarly, the average DPKG-based system has two or three package managers by default (Synaptic, Aptitude, apt-get, dselect, and so on). I don't know enough about Pacman to comment there.
The front-end is really not all that interesting - there's a degree of dependency resolution it needs to handle, but the really "difficult" part of package management comes from the underlying package format.
Again, I can't comment on Pacman, but between RPM and DPKG, It's RPM which has a tougher job - you can define extremely complex relationships in your RPM package, which the resolver needs to somehow handle, and a single slip-up in your package definitions may cause misery down the line. DPKG is a much simpler format as far as dependency information is concerned, since you can only define two things - package names and versions - whereas with RPM you can do complicated things like versioned symbols in dependencies, and so on.
So, to answer your question, the choice between managers within a single format is personal taste only (I mostly use aptitude). The choice between formats is one of how fine-grained a control you want (and, ironically, how much complexity you want that control to introduce into your packaging).